During scanning of a recorded optically detectable data track the alignment of the detector with the data track has been controlled by adjacent, parallel, optical servo tracks or marks, the sensing of which directs a servo motor to retain a detector on the data track. In U.S. Pat. No. 3,739,154, the skew position of a linear array transverse to the multiple data tracks being read simultaneously, is detected by the sequence of the passage of the track centers past the individual detectors in the array. The tracking error signal is delivered to a servo to correct the track position at the start of the scan. Logic and switching circuits are employed to correct for tracking errors and provide readout of the same track during the multiple track scan.
In U.S. Pat. No. 4,337,394, detection of parallel solid or broken lines and bars on a support plate give indication and direction of skew position of the perpendicularly disposed solid state line image sensor of a line-by-line image dissecting system.
In U.S. Pat. No. 3,700,862, the skew of multiple magnetic-type reading heads relative to transversely located multiple data tracks is detected by the alignment of the pulses from parallel data and clock tracks running in pairs of the upper and lower longitudinal margins of the card.
When data tracks are placed in rows perpendicular to the direction of motion of the recording medium material, it is possible to image the data in the entire row without the need for track following. However, the detector must be aligned with the data track during movement of the recording medium below the detector. If the detector is skewed beyond a certain limit is is possible that the detector will incorrectly read a part of an adjoining track.
It is therefore the object of the invention to device a means for detecting both direction and angle of skew of a linearly scanning detector relative to rows of data tracks aligned perpendicular to the direction of travel of the recording medium.